1. Field of the Invention
This invention relates to novel nonlinear-active copolymer and electrooptical and photonic components constructed therefrom.
2. Description of the Related Art
Polymers having nonlinear-optical properties are known and used, for example, as electrooptical switches, and are employed in areas of information processing and of integrated optics, such as optical chip-to-chip connections, waveguiding in electrooptical layers, Mach-Zehnder interferometers, and optical signal processing in sensor technology.
An overview of current problems in the development of materials having pronounced nonlinear-optical (NLO) properties is given in Angewandte Chemie, Vol. 107 (1995), pages 167 to 187. In addition to the requirements which are necessarily made of nonlinear-optical chromophores, reference is made to the problems in the development of polymeric matrices for the embedding of the chromophores and for their orientation-stable alignment.
So that polymers which are provided with covalently bonded or dissolved nonlinear-optical chromophores become nonlinear-optically active and exhibit high 2nd-order susceptibility, the chromophores must be oriented in an electrical field. In this context, reference is made to J. D. Swalen et al. in J. Messier, F. Kajzar, P. Prasad "Organic Molecules for Nonlinear Optics and Photonics", Kluwer Academic Publishers 1991, pages 433 to 445. This orientation usually takes place in the region of the glass transition temperature, where the mobility of the chain segments of the polymers allows orientation of the nonlinear-optical chromophores. The orientation obtained in the field is then frozen in by cooling or, better still, by crosslinking of the polymer. The 2nd-order susceptibility X.sup.(2) that can be achieved here is proportional to the spatial density of the hyperpolarizability .beta., to the ground-state dipole moment .mu..sub.0 of the chromophores, to the electrical poling field, and to parameters which describe the distribution of orientation after the poling process. In this context, reference is made to K. D. Singer et al. in P. N. Prasad, D. R. Ulrich "Nonlinear Optical and Electroactive Polymers", Plenum Press, New York 1988, pages 189 to 204.
Great interest attaches to compounds which combine a high dipole moment with high values of .mu.. Consequently, investigations have been carried out in particular into chromophores consisting of conjugated n-electron systems which carry an electron donor at one end of the molecule and an electron acceptor at the other end and are bonded covalently to a polymer: for example, to polymethyl methacrylate (U.S. Pat. No. 4,915,491), polyurethane, (EP-A 0 350 112) or polysiloxane (U.S. Pat. No. 4,796,976).
The above-mentioned polymer materials, known in the prior art, which have nonlinear-optical properties possess the disadvantage that there is relaxation of the oriented chromophore units and thus a loss of the nonlinear-optical activity. At present, this relaxation is still preventing the production of electrooptical components of long-term stability that are deployable technically.
A further disadvantage of the known polymer materials having nonlinear-optical properties is that it is impossible to modify the magnitude of the NLO coefficient and other important parameters, such as refractive index and glass transition temperature. Furthermore, owing to their chemical structure, the chromophoric systems described to date lack sufficient thermal stability to withstand without damage the thermal stresses which occur during the production and/or use of the electrooptical and photonic components. For instance, even at 85.degree. C. there is a marked drop in the measured electrooptical coefficients as a result of the relaxation processes of the chromophores in the polymer matrix.
What would be desirable would be stability of these optical coefficients at temperatures above 100.degree. C., and for this reason there is a simultaneous requirement for a substantially higher glass transition temperature of the polymeric material.